Apparatus and optical method of ranging and of high bit-rate communication

ABSTRACT

An optical apparatus for ranging and communication in free space comprises a rangefinder comprising a device for transmitting an optical signal to a target and a device for receiving the signals backscattered by the target. A system for optical communication in free space comprises a device for transmitting an optical signal to a remote optical receiving device. The transmitting device of the rangefinder and transmitting device of the communication system is a transmitting device common to the rangefinder and communication system and transmitting pulses of peak power greater than 50 W and shape factor less than 0.01 or a modulated continuous signal of peak power less than 10 W and shape factor equal to approximately 0.5 and the apparatus comprises a supervisor controlling the common transmitting device according to two modes, the pulse mode to perform the ranging function, or the modulated continuous mode to perform the optical communication function.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International patent applicationPCT/EP2011/070428, filed on Nov. 18, 2011, which claims priority toforeign French patent application No. FR 1004817, filed on Dec. 10,2010, the disclosures of which are incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The field of the invention is that of apparatus performing the functionsof optical ranging and of optical communication in free space. Morespecifically, it involves an apparatus of which the range of theairborne rangefinder in air-air can reach up to several tens ofkilometers and performing communications in free space at very high bitrate, of the order of several gigahertz.

BACKGROUND

A rangefinder is used to measure the distance separating it from atarget. An optical rangefinder uses the propagation of the light as ameasurement means. It comprises a transmitter and a receiver. Ittransmits light toward the target and detects a fraction of this lightreturned by the target. The distance is obtained based on thepropagation time back and forth of the light from the transmitter to thereceiver. The transmission is modulated in time. The transmitted lighttransports this modulation to the target. The light is absorbed by theatmosphere along the outward path. It is then absorbed and reflected orbackscattered by the target and then absorbed by the atmosphere on thereturn path; it is diluted along the return path by a factorproportional to the square of the distance. A fraction of this returnedlight transports the modulation to the receiver of the rangefinder. Thistime modulation makes it possible to identify the start of the pulse andidentify its return by the receiver. The elapsed time between these twoevents makes it possible to calculate the distance between therangefinder and the target based on the speed of propagation of thelight in the media that are passed through.

When the distance increases, the quantity of light detected decreasesrapidly. To increase the ranging distance despite these atmosphericlosses, the following ways are possible:

-   -   increasing the energy per pulse, but this increase is limited by        the constraints of ocular safety and by the volume of the        transmitter which increases with the energy per pulse,    -   increasing the dimension of the receiving pupil but this        increases the dimensions of the system,    -   increasing the sensitivity of the receiver with multipulse        systems using micro-lasers or fiber optic lasers. This makes it        possible to use post-integration. There is increase in the mean        power (energy per pulse×rate) without increasing the energy per        pulse.

Currently there are three main categories of laser rangefinders.

-   -   Rangefinders having a modulated continuous transmission    -   Multipulse rangefinders    -   Monopulse rangefinders

The rangefinders having a modulated continuous transmission are usedwith cooperative targets of which the measurement time is not critical.A cooperative target is for example fitted with a back reflector, andtherefore returns the light in a narrow cone in the direction of thetransmitter. The system is designed so that reception is possible andduring transmission.

For noncooperative targets situated at long distances of the order ofseveral tens of km, the rangefinders usually use a single pulse of greatenergy limited by ocular safety in the conditions of use: the integratedexposure over 10 seconds, for a wavelength of between 1.5 and 1.8 μm,must remain below 10 000 J/m². This limit, depending on theapplications, allows energies per pulse of from a few millijoules toseveral tens of millijoules. To achieve good distance accuracy, thepulses have a very short duration: of the order of 10 ns. Detection ofthe echoes is not possible during the transmission of the pulses.

For short distances (<10 km), it is possible to use laser diodes as atransmitter. The energy per pulse is very low. The performance isobtained by multiple pulses with detection with post-integration. Thepulse duration of the order of 10 to 50 ns is very low compared with theperiod between the pulses which is of the order of 1 to 50 μs. Duringtransmission, reception is blind. The diffusion of the transmitted lightby the atmosphere over a short distance (from a few meters to a few tensof meters) blinds reception. Beyond this, detection takes place duringthe period between the pulses. Detection of the echo is a detection ofenergy.

Post-integration has certain drawbacks.

Specifically, note that:

if, for a transmitted pulse, there is for the echo a signal-to-noiseratio S/B,

therefore n transmitted pulses gives (nS)/(n^(1/2) B), or (n^(1/2) S)/B,hence an improvement of factor n^(1/2).

But in the case of post-integration, the frequency of repetition of thepulses (or rate) limits the distance that can be achieved because of theambiguity concerning the distance. This ambiguity occurs when a detectedpulse originates either from the last transmitted pulse, returned by aclose target, or from a pulse transmitted earlier and returned by adistant target, without it being possible to determine between these 2alternatives which target is measured. By accepting a larger timescaleof blind reception, each pulse can be replaced by a pulse train.

An optical system for high bit-rate communication in free space alsocomprises a laser transmitting device for an optical signal and if thecommunication is two-way, it also comprises a device for receiving theoptical signals transmitted by another communication system. The opticalsignal transmitted is a rapid succession of pulses at a period ofrepetition typically between 1 ns and 20 ms. The gaps between the pulseshave periods similar to the pulse widths. Digital data consist of 0s and1s. Each data bit is associated with a unitary period: a pulse duringthis unitary period represents a 1, no pulse during this periodrepresents a 0. The data sequences are also usually encoded bysuccessions of pulses and by periods between the pulses. The peak powerof the communication pulses is on average double the mean power of thecommunication transmission. The transmission is of the modulatedcontinuous type on two levels 0 and 1. In the rest of the description,such a succession of pulses modulated in this way is called the opticalcommunication signal. Several examples (16 examples) of high bit-ratecommunication signals are shown in FIG. 3 b. Over the first 10 ns, theexample of the 4^(th) channel corresponds to the following digitalsequence: 00100110010.

The transmitting device of the rangefinder and that of the communicationsystem therefore obey contradictory constraints, and their receivingdevices. Hence the use of two independent devices for performing thefunctions of long-range ranging and of high bit-rate opticalcommunication in free space.

Such apparatus are then bulky and heavy. The object of the invention isto alleviate these drawbacks.

SUMMARY OF THE INVENTION

The solution according to the invention is based on the use of a singlelaser transmitting device that can operate in two different modes, inpulse mode, promoting energy per pulse, for ranging and in linearmodulated continuous mode for high bit-rate optical communications.

More precisely, the subject of the invention is an optical apparatus forranging and communication in free space which comprises a rangefindercomprising a device for transmitting an optical signal to a target and adevice for receiving the signals backscattered by the target, and asystem for optical communication in free space comprising a device fortransmitting an optical signal to a remote optical receiving device. Itis mainly characterized in that the transmitting device of therangefinder and the transmitting device of the communication system is atransmitting device common to the rangefinder and to the communicationsystem and capable of transmitting pulses of which the peak power isgreater than 50 W and the shape factor is less than 0.01 or a modulatedcontinuous signal of which the peak power is less than 10 W and theshape factor is equal to approximately 0.5 and in that the apparatuscomprises a supervisor (1) capable of controlling the commontransmitting device according to two modes, the pulse mode in order thusto perform the ranging function, or the modulated continuous mode inorder to thus perform the optical communication function. The shapefactor is defined as: a ratio of peak power over mean power of thepulses; and for a digital signal composed of 1 and 0, this ratio of peakpower over mean power=the duration of the ‘1’ signals/duration of the‘1’ and ‘0’ signals for optical communications.

This apparatus makes it possible to perform both functions of long-rangeranging and of high bit-rate optical communication with a singleapparatus. It is flexible to use with an instantaneous transition fromone mode to the other, which allows the interleaving of the two modes aswill be seen below.

According to one feature of the invention, the common transmittingdevice comprises a laser diode transmitter which comprises an electricalpower supply, and the supervisor comprises means for controlling theelectrical power supply of the laser diode transmitter.

This laser diode transmitter may be a single-ribbon laser diode or astack of single-ribbon diodes capable of transmitting collectively.

Preferably, the transmitting device comprises a transmitter and anamplifier connected to the output of the transmitter.

According to one feature of the invention, the apparatus comprises adevice for receiving signals transmitted by another opticalcommunication device in free space, the receiving device of therangefinder and this receiving device of the communication system beinga common receiving device, and the apparatus comprises a control of thereceiving device in ranging mode or in communication mode.

The supervisor comprises for example the control of the receivingdevice.

The transmitting device and receiving device are advantageouslymultiwavelength.

According to one feature of the invention, the multiwavelengthtransmitter comprises several transmitters, each being capable oftransmitting at a different wavelength from the others and it comprisesa single wideband amplifier connected to all these transmitters.

When the receiving device is multiwavelength, at least certain receptionwavelengths are identical to certain transmission wavelengths.

A further subject of the invention is a ranging method of a target bymeans of an optical apparatus for ranging and communication as describedabove, which comprises a step of transmitting laser pulses to the targetby means of the common transmitting device and a step of receiving thepulses backscattered by the target by means of the device for receivingthe signal backscattered by the target. It is characterized in that italso comprises a step of transmitting by means of said commontransmitting device a modulated continuous optical communication signalto a device for receiving this communication signal, the step oftransmitting a communication signal being carried out outside theranging transmitting and receiving steps.

According to one feature of the invention, it comprises several steps oftransmitting a communication signal, and the step of transmitting laserpulses to the target, the step of receiving the pulses backscattered bythe target and these steps of transmitting a communication signal areinterleaved so that an optical communication signal is transmittedbetween two pulses and outside the step of receiving the pulsesbackscattered by the target.

According to another feature of the invention, since the transmittingdevice comprises a single amplifier, the time gap between twoconsecutive ranging pulses or between the end of a communication signaland the consecutive ranging pulse is greater than or equal to the timefor pumping the amplifier to saturation.

When the transmitting device is multiwavelength and comprises only asingle wideband amplifier, the time gap between two ranging pulses thatare of different wavelength and consecutive is greater than or equal tothe time for pumping the amplifier to saturation, the time gap betweenthe end of a communication signal and the consecutive ranging pulse ofthe same wavelength is greater than or equal to the time for pumping theamplifier to saturation, and the communication signals of differentwavelength are transmitted at the same time.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become evident onreading the following detailed description, made as a nonlimitingexample and with reference to the appended drawings in which:

FIG. 1 represents schematically an example of a transmitting device andof a receiving device common to the rangefinder and to the communicationsystem in a monowavelength configuration with an amplifier,

FIG. 2 represents schematically an example of a transmitting device andreceiving device common to the rangefinder and to the communicationsystem in a multiwavelength configuration with a single widebandamplifier,

FIG. 3 illustrate schematically examples of moments of transmission ofpulses in ranging mode (FIG. 3 a), in communication mode (FIG. 3 b) andin interleaved modes (FIGS. 3 c and 3 d) for a multiwavelengthapparatus.

From one figure to the next, the same elements are indicated by the samereferences.

DETAILED DESCRIPTION

The optical apparatus for ranging and communication can bemonowavelength or multiwavelength.

Consideration is given first of all to a monowavelength apparatus.

With reference to FIG. 1, a transmitting device 10 that is common to therangefinder and to the communication system will be described. Itcomprises a laser source 11 that can be connected at the output to anamplifier 12 such as a fiber amplifier.

First, the case of a laser diode transmitter that is not connected atthe output to an amplifier is envisaged.

The laser diode transmitter can be a single-ribbon laser diode or astack of single-ribbon diodes transmitting collectively.

The pulse width is produced by the power supply of the laser diode or bythat of the stack. Beyond a threshold, the transmission power isproportional to the power supply current.

In virtually continuous operation for communication, the maximum currentand therefore the mean power are mainly limited by the thermal behaviorof the component.

In ranging, with very short pulses, the peak power can be much greaterand reach approximately 30 times the mean power used in virtuallycontinuous operation. There is no thermal limitation as above, but alimitation by the resistance to the optical flux of the faces of thelaser diode components.

The transmission time profile is very different for ranging andcommunication. It is via the electrical control of the laser diode thatthe two functions are differentiated depending on the desired timeprofile and the possible maximum current.

By taking as an example a simple laser diode, for the ranging function,pulses are obtained that have a peak power of several tens of watts anda repetition frequency of 20 to 30 kHz, by causing the current of thediode to vary from 0 to 10 A, at this same repetition frequency. For thecommunication function, a modulated optical signal is obtained at afrequency of several MHz and having a mean power of approximately 100mW, by causing the current of the diode to vary from 0 to 100 mA at thissame frequency.

A laser diode transmitter followed by an optical amplifier is nowenvisaged.

The optical amplifier 12 makes it possible to increase the ranging andcommunication performance. In ranging, this amplifier makes it possibleto increase the energy of the pulse transmitted. In communication, it isthe optical power that is mainly increased by the amplifier.

In ranging, the pumping of the amplification between two pulses mustallow each pulse to have the required energy. The time gap between twopulses is adjusted so that the amplifier is pumped to saturation beforea new pulse is transmitted by the laser diode. When the pulse istransmitted, it is amplified. This amplification depends on the energyof the pulse at the input of the amplifier. When the energy of theoptical pulse of the laser diode is sufficient for all the energy storedin the amplifier to be transferred in the transmitted pulse, thetransmitted pulse is then amplified to the maximum. The amplifier usedis designed to supply high energies per pulse. The energy per pulse maybe of the millijoule class. Such amplifiers are available from Manlight(Luskenn product class) or Keopsys (EOLA product class).

In communication, the laser diode transmits the pulse train of thecommunication signal while the amplifier is pumped; between two pulsesof this pulse train, the amplifier does not have the time to reach thesaturated mode. The mean power level of the laser diode is adjusted tooptimize the efficiency of the device, which is to say so that all thepulses of the communication signal are amplified to the maximum all withthe same gain.

For reasons of optimization, since the amplifier 12 is common to bothfunctions, ranging and communication, circuits placed between the laserdiode 11 and the amplifier 12 can be added, one for ranging and theother for communication. The circuit for ranging is typically apreamplifier to increase the energy of the pulse and thus promote theextraction of the energy of the amplifier; the circuit for communicationmay also be a preamplifier which has the function of adjusting theenergy of each pulse in a predetermined range. The same preamplifier maybe used for ranging and for communication provided that it is adaptedaccording to its use as indicated above.

For a transmitter using an amplifier, the ranging and communicationfunctions are possible sequentially; they may also be interleavedchronologically. As has been seen above, it is necessary in ranging thatthe amplifier 12 has its full energy capacity for each pulse. After thetransmission of a ranging pulse, the amplifier needs sufficient pumpingtime to have the expected gain to amplify a communication pulse train.Similarly, after amplification of the communication pulse train, sometime is necessary for the amplifier to recover its full capacities forthe amplification of the pulses for ranging. This constraint is the samefor a monowavelength transmitter or multiwavelength transmitter.Moreover, the communication transmission is not possible during the waitfor the ranging echo.

The receiving part of a monowavelength apparatus will now be explainedin detail. It comprises a receiving device for ranging and, in the caseof a two-way communication system, it also comprises a receiving devicefor communication. These receiving devices may be independent of oneanother.

The receiving device for ranging comprises schematically a lens forcollecting the light coming from the target, which focuses it on adetector. There may be a conveyance by fiber between the focusing pointof the collecting lens and the detector. Devices for spectral filteringand for separating the transmitting and receiving channels may also beinterposed. An optical amplification or a transposition of wavelengthmay also be in the path before the photodetector. The receivingphotodetector may be a PIN or avalanche photodiode. The electricalbandwidth of the transimpedance circuit associated with thisphotodetector is adapted to the width of the pulses to be detected whichis typically between 10 and 50 ns.

In the case of ranging, the extraction of a pulse is sought in the noisefor the period corresponding to the extent of distance at which thetarget may be. According to the monopulse or multipulse operating mode(and therefore with post-integration), analog and digital processesknown to those skilled in the art can be used to determine thedistances. The limit of the processes is the rate of false alarms,namely how many false distances are fed back as a function of the numberof tests.

Improving the performances consists in detecting the weakest possiblesignals by controlling all the noises associated with detection. Thesources of noise are optical and electronic.

The receiving device for communication comprises schematically a lensfor collecting the light that is transmitted directly by the remotetransmitter of another communication system, focusing it on a detector.There may be a conveyance by fiber between the focusing point of thecollecting lens and the detector. Devices for spectral filtering and forseparating the transmitting and receiving channels may also beinterposed. An optical amplification or a transposition of wavelengthmay also be in the path before the photodetector. The receivingphotodetector for communication may be a PIN or avalanche photodiode.The detector may be a single element which means a single communicationchannel; this single element may be capable of detecting withoutdistinction one or more wavelengths.

The electrical bandwidth of the transimpedance circuit associated withthe photodetector is adapted to the bit rate that is higher than inranging; it is greater than 100 MHz.

Communication performance depends on the bit rate that is acceptable bythe transmitter, the receiver and the detectable energy.

In the case of communication, at each moment the processing must discerna 1 or a 0. The limit of the processes is the error rate.

Improving the performances consists in increasing the bit rate oftransmitted information. The bit rate in monowavelength increases withthe frequency of the pulse train. For communication to be againdetected, the bandwidth must be increased to remain suitable.

The reception of communication is lit directly by the remotetransmitter. Depending on the distance, the receiving conditions and thedesign of the receiver, the energy per bit and the frequency ofcommunication are limited.

According to a particular embodiment of the invention, the receivingdevice for ranging is not independent from that of the receiving systemfor communication. Depending on the expected bit-rate performances ofcommunication, the ranging detection and the communication detectionshare all or some of the necessary elements. The receiving device 20 ofthe apparatus may be common to ranging and to two-way communication. Thetransimpedance circuit associated with the photodetector may be thesame. But it may be necessary to have detection with two modes havingone bandwidth adapted to ranging and another bandwidth adapted tocommunication. The detection signal is then processed according to itsuse, ranging or communication.

The processings of the data originating from ranging and fromcommunication are different notably because of the very different bitrates. Ranging is limited to a few pulses received per second.Communication may have bit rates of from kilobits to gigabits persecond.

This common receiving device 20 can operate in ranging or incommunication. This dual function makes it possible to range the targetand transmits information to it.

For a diode transmitter with a single wavelength, it is risky to have acommunication transmission when the rangefinder is waiting for rangingechoes. The diffusion of the light transmitted may disrupt the rangingreception. The two functions are possible sequentially.

FIG. 1 shows an example of an apparatus for ranging and monowavelengthcommunication and ranging according to the invention. It comprises asupervisor 1 capable of receiving the communication or ranging data andof transmitting them to the common transmitting device 10 by sequencingthe ranging transmitting and receiving steps and the communicationtransmitting and optionally receiving steps. The common transmittingdevice 10 comprises a laser diode 11 optionally connected at the outputto an amplifier 12 itself connected to a optical device 13 for shapingthe transmitted beam. The common receiving device 20 comprises a lens 23for collecting the light transmitted by the target or by a remotecommunication transmitter; this lens 23 is optionally connected to anamplifier 22 itself connected to a photodetector 21 which transmits thedetected signal to a processing unit 24 capable of supplying at theoutput distances of targets and communication data depending on whetherthe supervisor 1 controls this processing unit 24 in ranging orcommunication mode.

Advantageously, the apparatus according to the invention ismultiwavelength, which makes it possible to increase the range ofranging and the number of codes for high bit-rate communications. It isdescribed with respect to FIG. 2.

Consideration is first given to the transmitting device 10 common to therangefinder and to the communication system.

In the case of a multiwavelength system, there are as many laser diodes111, 112, 113, 114 as wavelengths to be transmitted. In the case ofusage of an amplifier 12, it is common to all the wavelengths. Beforethe amplifier, the transmissions of the laser diodes are superposed in asingle beam. This can be done by mirrors or by fiber coupling. Theoutput is common to all the transmissions.

In ranging mode, each wavelength is transmitted on its own in order tohave the strongest energy content. The advantage of the multiwavelengthis the possibility of increasing the repetition frequency of the pulseswith no problem of distance ambiguity. Specifically, this is theequivalent of having N rangefinders in parallel, each having a fairlylarge distance ambiguity relative to the range intended for ranging. Inthis pulse mode, the repetition frequency of the pulses is below thethreshold frequency. Thus, no transmitter encounters the problem ofambiguity over the distance. From one wavelength to another, therepetition frequencies of the pulses may be different or identical. Themoments of transmission of the ranging pulses of different wavelengthsare preferably different because of the energy necessary for each pulsewhich otherwise would be shared; in FIG. 3 a, N wavelengths (N=16 in thefigure) are transmitted in succession and the repetition frequency isthe same from one wavelength to another, in this instance 400 μs for adistance of ambiguity for targets from 60 km. The trains of N pulses arerepeated many times in order to obtain the range budget bypost-integration. The sequence of pulses is therefore: λ1, . . . , λN,λ1, . . . , λN, etc.

Since the amplifier is common to all the wavelengths, the pumping of theamplification between two pulses of different wavelength must allow eachpulse to have the required energy. In the example of FIG. 3 a, the gapbetween two pulses of different wavelength is 25 μs; pumping thereforelasts a maximum of 25 μs.

The order of the wavelengths has no effect on the ranging range.

The order of wavelengths may carry information and be used for otherpurposes, for identification for example.

Operation in communication depends on the capacity of the remotereceiver for which this communication is intended. If this remotereceiver has a single detector, only one communication channel can beset up. The remote receiver may be capable of receiving differentwavelengths. The transmitting device must transmit at a wavelength thatcan be detected by the remote receiver. If the reception wavelength isunknown, the transmission may also activate in parallel severalwavelengths transmitting identically and synchronously.

Communication may also be dedicated to a class of receiverscharacterized by its capacity to receive certain wavelengths. Discretionmay be provided by a choice of wavelength beyond those used by mostrangefinders.

For communications, the N wavelengths are used simultaneously and carryindependent codes (typically N=16). And one code is associated with eachwavelength. This code consists, for example, in modulating the rate ofthe pulses which varies from one wavelength to another as can be seen inFIG. 3 b. Each wavelength is an information channel independent of theothers. The transmission will independently activate each wavelengththat can be detected by the remote receiver. The amplification ontransmission may be common to all the wavelengths. In this case, thecommunication signals which are usually different from one channel toanother are simultaneously transmitted at all the wavelengths becausethere can be no ranging pulse transmission during the transmission of acommunication signal. This makes it possible to transmit as manyindependent communication channels. The bit rate of communication is theproduct of the number of channels times the bit rate of each channel.Some of these channels may be exclusively dedicated to ranging.

FIG. 3 b shows the operation of 16 channels of simultaneouscommunication at 1 gigahertz each, or a total bit rate of the order of16 gigabits/second.

The common transmitting device can operate in ranging and incommunication. This dual function makes it possible to rangefinder thetarget and to transmit information to it.

The multiwavelength apparatus can also operate with both modesinterleaved. This dual mode is illustrated in FIG. 3 c, for aconfiguration of the transmitting device with a single amplifier for allthe wavelengths. The amplifier remains in saturated mode whiletransmitting a pulse for each wavelength with moments of transmissionshifted from one wavelength to another, and then switches to continuousmode during the transmission for each wavelength of the modulatedsignal, these transmissions being simultaneous. Seen from the amplifier,the example of FIG. 3 c is projected chronologically on the time axis inFIG. 3 d: the amplifier has a pumping time of approximately 25 μsbetween two amplifications of ranging pulses of different wavelength, orbetween the amplification of one ranging pulse and that of the 1^(st)pulse of the communication pulse train. Because of these constraintsstemming from the amplification, for each wavelength, communicationtransmission is not possible while awaiting ranging echoes of this samewavelength.

FIG. 3 c represents a ranging and communication operating option using16 wavelengths. The operating mode shown makes it necessary toneutralize the ranging receivers during the communication transmission.The system would have in ranging an efficacy equivalent to the use ofthe order of 14 to 15 wavelengths: specifically, it is possible that forone (or two) wavelength(s) the communication signal is transmittedduring the reception of the corresponding ranging echo.

Depending on the performance requirements of ranging and ofcommunication, many other ranging and communication sequencings as afunction of the wavelengths can be optimized.

The reception portion of a multiwavelength apparatus is now considered.

The receiving device for ranging preferably comprises a device foramplifying each reflected pulse which makes it possible to amplify veryweak signals by adding the minimum noise to them: the objective is toimprove the detection signal-to-noise ratio. As for transmission, it ispossible to choose a wideband amplification device which amplifieswithout distinction each pulse and/or narrowband amplification deviceswhich amplify specifically each pulse.

An optical filter of the reflected pulses coming from the target is usedto separate the various channels to be detected, that is to say in orderto demultiplex them. The separation is spectral.

After this filtering, it is possible to amplify the pulses of eachchannel by means of a narrowband amplifier.

A detector can be used only in relation with a single channel. Since thedistance of the target is not known, detection must be active so long asechoes are expected. Detection may be obtained by a detector capable ofdetecting the reflected pulses for several wavelengths and ofdistinguishing them according to their wavelength. There are no clearadvantages in range performances over the monowavelength system. Whenlong ranges are sought, there is an advantage in having as manydetectors as transmission wavelengths.

A conventional accumulation device is used to carry out thepost-integration of the detected pulses.

Operation in communication depends on the capacity of the receiver.

The receiving device for communication may have several detectors indifferent wavelengths. Each wavelength is an information channelindependent of the others.

The multiwavelength receiving device can also operate with both modesinterleaved as in the case of the monowavelength receiving device.

FIG. 2 shows an example of a multiwavelength apparatus for ranging andcommunication according to the invention. It comprises a supervisor 1capable of receiving the communication data or the ranging data and oftransmitting them to the common transmitting device 10 by sequencing theranging transmitting and receiving steps. The transmitting devicecomprises a wavelength distributor 14 connected at the input to thesupervisor 1 and at the output to laser diodes 111, 112, 113, 114 ofdifferent wavelength λ1, λ2, λ3, λi. The common transmitting deviceoptionally also comprises these diodes, an amplifier 12 connected at theinput to each of the diodes and at the output to an optical device 13for shaping the transmitted beam. The common receiving device 20comprises a lens 23 for collecting the light transmitted by the targetor by a remote communication transmitter; this lens is optionallyconnected to an amplifier 22 itself connected to photodetectors 211,212, 213, 214 respectively dedicated to a different wavelength λ1, λ2,λ3, etc. which respectively transmit their detected signal to aprocessing unit 24 capable of providing at the output distances oftargets and communication data depending on whether the supervisor 1controls this processing unit in ranging mode or communication mode. Thephotodetectors are connected via another distributor 25 to thesupervisor 1.

The invention claimed is:
 1. An optical apparatus for ranging andcommunication in free space comprising: a rangefinder comprising atransmitting device for transmitting an optical signal for ranging to atarget and a receiving device for receiving the signals backscattered bythe target, an optical communication system for optical communication infree space comprising the transmitting device for transmitting anoptical signal for optical communication to a remote optical receivingdevice, wherein the transmitting device of the rangefinder and thetransmitting device of the optical communication system is common to therangefinder and to the optical communication system and capable oftransmitting pulses of which a peak power is greater than 50 W and ashape factor is less than 0.01, the shape factor being defined as aratio of the peak power over a mean power of the pulses, or a modulatedcontinuous signal of which the peak power is less than 10 W and theshape factor equal to approximately 0.5, and a supervisor capable ofcontrolling the common transmitting device according to two modes, apulse mode to perform the ranging, or a modulated continuous mode toperform the optical communication.
 2. The optical apparatus for rangingand communication as claimed in claim 1, wherein the common transmittingdevice comprises a laser diode transmitter including an electrical powersupply, and the supervisor comprises means for controlling theelectrical power supply of the laser diode transmitter.
 3. The opticalapparatus for ranging and communication as claimed in claim 1, whereinthe laser diode transmitter is a single-ribbon laser diode or a stack ofsingle-ribbon diodes capable of transmitting collectively.
 4. Theoptical apparatus for ranging and communication as claimed in claim 1,wherein the transmitting device comprises a transmitter and an amplifierconnected to an output of the transmitter.
 5. The optical apparatus forranging and communication as claimed in claim 1, wherein the receivingdevice is configured to receive signals transmitted by another opticalcommunication device in free space, the receiving device being a commonreceiving device to the rangefinder and the optical communicationsystem, and the apparatus comprises a control of the receiving device inthe pulse mode for ranging or in the modulated continuous mode foroptical communication.
 6. The optical apparatus for ranging andcommunication as claimed in claim 5, wherein the supervisor comprises acontrol of the common receiving device.
 7. The optical apparatus forranging and communication as claimed in claim 1, wherein the commontransmitting device and the common receiving device are multiwavelength.8. The optical apparatus for ranging and communication as claimed inclaim 7, wherein the multiwavelength transmitter comprises a pluralityof transmitters, each being capable of transmitting at a differentwavelength from the others and comprising a single wideband amplifierconnected to all of the plurality of transmitters.
 9. The opticalapparatus for ranging and communication as claimed in claim 1, whereinthe common receiving device is multiwavelength, at least one receptionwavelength being identical to one transmission wavelength.
 10. A rangingmethod of a target by means of an optical apparatus for ranging andcommunication as claimed in claim 1, comprising: a step of transmittinglaser pulses to the target by the common transmitting device, and a stepof receiving the laser pulses backscattered by the target by thereceiving device for receiving the signal backscattered by the target,and a step of transmitting a modulated continuous optical signal to adevice for receiving the modulated continuous optical signal by saidcommon transmitting device, the step of transmitting the modulatedcontinuous optical signal being carried out when the step oftransmitting the laser pulses and the step of receiving the laser puslesis not being carried out.
 11. The ranging method of a target as claimedin claim 10, wherein the transmitting device includes a singleamplifier, a time gap between two consecutive ranging pulses or betweenan end of a communication signal and a consecutive ranging pulse isgreater than or equal to a time for pumping the amplifier to saturation.12. The ranging method of a target by means of an apparatus as claimedin claim 10, wherein the common transmitting device is multiwavelengthand includes only a single time wideband amplifier, a time gap betweentwo ranging pulses that are of different wavelength and consecutive isgreater than or equal to a time for pumping the wideband amplifier tosaturation, a time gap between an end of a communication signal and aconsecutive ranging pulse of the same wavelength is greater than orequal to the time for pumping the wideband amplifier to saturation, andthe method comprising transmitting communication signals of differentwavelength at the same time.
 13. The ranging method of a target asclaimed in claim 1, further comprising a plurality of steps oftransmitting a communication signal, and the step of transmitting laserpulses to the target, the step of receiving the laser pulsesbackscattered by the target and the plurality of steps of transmittingthe communication signal are interleaved so that an opticalcommunication signal is transmitted between two laser pulses and whenthe step of receiving the laser pulses is not being carried out.